Chapter 4 : Sporulation in Bacteria: Beyond the Standard Model

Bacteria that produce endospores or intracellular offspring exhibit a wide variety of morphological phenotypes. (A) Phase-contrast microscopy is often used to identify mature endospores (A to C and E) as these highly mineralized cells appear phase-bright. In this image of B. subtilis, the caret (>) indicates a cell that is not dividing or sporulating and the asterisk (*) indicates a cell undergoing binary fission. All other cells in the image contain a phase-bright endospore. (B) Clostridium oceanicum frequently produces phase-bright endospores at both ends of the cell. Image courtesy of Avigdor Eldar and Michael Elowitz, California Institute of Technology. (C) In this image of Anaerobacter polyendosporus, the arrows indicate cells with seven endospores. (D) The fluorescence micrograph of Metabacterium polyspora outlines cell membranes and spore coats stained with FM1-43. (E) Epulopiscium-like type C (cigar-shaped cell) and type J (elongated cells), each containing two phase-bright endospores. (F) Epulopiscium sp. type B with two internal daughter cells, stained with DAPI. Cellular DNA is located at the periphery of the cytoplasm in the mother cell and each offspring. (G) Scanning electron micrograph (SEM) of the ileum lining from a rat reveals the epithelial surface densely populated with SFB. Arrow indicates a holdfast cell that has not yet elongated into a filament. (H) Transmission electron micrograph (TEM) of a thin section through the gut wall reveals the structure of the SFB holdfast cell (indicated by an asterisk). (I to J) TEMs illustrate the two possible fates for developing intracellular SFB: (I) two holdfast cells or (J) two endospores that are encased in a common coat (C), inner (I) and outer (O) cortex. Panel C reproduced from Siunov et al. ( 47 ) with permission from Society for General Microbiology. Panel E reproduced from Flint et al. ( 33 ) with permission from ASM Press. Panel F reproduced from Mendell et al. ( 93 ) with permission from the National Academy of Sciences, USA. Panels G and H reproduced from Erlandsen and Chase ( 69 ) with permission from the American Society for Nutrition. Panels I and J reproduced from Ferguson and Birch-Andersen ( 74 ) with permission from John Wiley and Sons.

Endospore development. In monosporic bacteria, complete division occurs at only one end of the developing sporangium (A), while bacteria that produce two endospores generally divide at both poles (B). In some lineages, such as the SFB and M. polyspora, engulfed forespores undergo division (not shown). Note that at least three chromosome copies are required to produce two viable endospores. Following endospore engulfment, cortex and coat layers develop, and upon endospore maturation, the mother cell lyses, releasing one (A) or two (B) endospores.

Life cycle of Metabacterium polyspora. Endospores germinate (A) and, during outgrowth, a cell may undergo binary fission (B) or immediately begin to sporulate by dividing at the poles (C). The forespores are engulfed (D), and the forespores may undergo binary fission to produce additional forespores (E). Forespores then elongate (F) and develop into mature endospores (G). Figure reproduced from Ward and Angert ( 52 ) with permission from John Wiley and Sons.

Life cycle of SFB and Epulopiscium sp. type B. (A) (i) The SFB life cycle begins with a holdfast cell that is anchored to the intestinal epithelia (not shown). (ii) Holdfast cells elongate and divide into primary segments as the filament grows. (iii) At the start of development, cells in the filament divide again to produce secondary segments. (iv) Next, secondary segments divide asymmetrically, and then engulfment of the smaller cell (in grey) occurs, in a manner similar to that of other endosporeformers. Development progresses from the free end of the filament toward the holdfast. (v) Each engulfed offspring cell then forms into a crescent shape (vi) and then divides to either form two holdfast offspring cells per segment (inset, top) or develop into an endospore via formation of a spore cortex and coat (inset, bottom). (B) (i) In Epulopiscium sp. type B, twin offspring form by division at both cell poles. Engulfment occurs (ii to iii) and offspring cells elongate (iv). The offspring cells begin to produce their own offspring before they are released from the mother cell (v).